Seminarium Fizyki Ciała Stałego

Cavity light-mattercoupling in solid state systems has been recently approaching the ultrastrong coupling regime [1-4], where the Rabi frequency Ω is comparable to the bare excitation frequency ω. We recentlydemonstrated a new platform to investigate ultrastrong coupling physics: the cyclotron transition of a 2DEG is coupled to an Au metasurface of THz split-ring resonators reaching  the ultrastrong couplingregime and showing record high values of the light-matter coupling ratio Ω/ω=0.58 [5]. I will present our recent advancesin this polaritonic system. We employ Nb-based superconducting complementary metasurfaces[6] achieving adiabatic modulation of the polaritonicstates through temperature tuning. With the same kind of cavities and a sample withn=4 quantum wells we observe a record-high  normalized coupling ratio of Ω/ω=0.89 [7] at a frequency of 300 GHz. For such valuethe polaritonic dispersion clearly deviates from the linearregime. I will discuss also an  high quality factor complementary THz metasurface based on Niobium thin film [8], which displaysnarrow resonance and Q factor higher than 50 at T=3 K in a strongly subwavelength volume ( V_cav/ lambda³ of the order of 10^-6). I will present new experimentalresults obtained measuring these metasurfaces at temperaturesas low as 20 mK, where Q factors as high as 120 are measured.Our measurements highlight the role of the residual normal state electrons at temperatureswell below the critical temperature T_C. [1] C. Ciuti, G. Bastard, and I. Carusotto, Phys. Rev. B 72, 115303 (2005).[2] T. Niemczyk, F. Deppe, H. Huebl, E. P. Menzel, F. Hocke, M. J. Schwarz, J. J. Garcia-Ripoll, D. Zueco, T. Hümmer, E. Solano, et al., Nature Physics 6, 772776 (2010).[3] G. Günter et al., Nature 458, 178 (2009). [4] Y. Todorov et al., Phys. Rev. Lett. 105, 196402 (2010). [5| G. Scalari, C. Maissen, D. Turcinkova, D.Hagenmuller, S. deLiberato, C. Ciuti, C. Reichl, D.Schuh, W. Wegscheider, M. Beck and J. Faist, Science 335, 1323 (2012)[6] G. Scalari, C. Maissen, S. Cibella, R. Leoni, P. Carelli, F. Valmorra, M. Beck, and J. Faist, New Journal of Physics 16, 033005 (2014).[7] C. Maissen, G. Scalari, F. Valmorra, M. Beck, S. Cibella, R. Leoni, C. Reichl, C. Charpentier, W. Wegscheider, J. Faist, Phys. Rev. B 90, 205309 (2014) [8] G. Scalari et al., Applied Physics Letters 105 (26), 2 61104 (2014).